The invention relates to a valve train of an internal combustion engine, having at least one basic camshaft on which a cam carrier having at least one valve-actuating cam is disposed in a rotationally fixed and axially displaceable manner, wherein the cam carrier has a tubular basic element which at least partially accommodates the basic camshaft, on which basic element at least one cam element of the cam carrier, in particular the valve-operating cam, is arranged. The invention furthermore relates to an internal combustion engine and a method of manufacturing a valve train.
Valve trains of the aforementioned type are known in the art. They are used for internal combustion engines, where the operating cycle of gas-exchange valves of individual cylinders of the internal combustion engine can be controlled to improve the thermodynamic property. The at least one cam carrier, which can also be referred to as a cam piece, is arranged on the basic camshaft in a rotationally fixed and axially displaceable manner. The cam carrier is displaced in the axial direction by an adjusting device, which includes a shift gate on the cam carrier and a fixedly arranged actuator, typically in a cylinder head of the internal combustion engine. The actuator has an extendable follower which can be brought into engagement with a helical or spiral groove of the shift gate. At least one valve-actuating cam having an eccentricity, which serves to actuate a gas exchange valve of internal combustion engine at a certain rotational angle of the basic camshaft, is associated with the cam carrier. The valve-actuating cam therefore rotates together with the basic camshaft, so that the gas-exchange valve of the internal combustion engine is operated at least once per revolution of the valve-actuating cam or its eccentricity. The valve-actuating cam preferably cooperates with a roller cam follower of the gas exchange valve by making direct contact therewith.
Preferably, several valve-actuating cams are provided which may be associated with different cam groups. The valve-actuating cams can now vary in the angular position, in the extent in a radial direction and/or in the eccentricity in the circumferential direction. By way of the axial displacement of the cam carrier, the cam carrier can be brought into at least two, for example, in a first and a second actuating position. In the first actuating position, the gas exchange valve is actuated by a first of the valve-actuating cams and in the second actuating position by a second of the valve-actuating cams that are assigned to the same cam group. By the displacement of the cam carrier, in particular the opening timing, the opening duration and/or the stroke of the gas change valve can be selected, in particular as a function of an operating state of the internal combustion engine.
Conventional cam carriers are integrally formed and are made of a metallic solid material, which is subjected to various manufacturing steps. The manufacturing steps include, for example, reaming an internal toothing of the cam support, grinding or turning cylindrical portions of the cam carrier used for rotational support, electron-beam hardening of a surface of the valve-actuating cam as well as gas nitration of surfaces in the area of the shift gate. These manufacturing steps require a not inconsiderable effort and associated costs. In addition, split bearings with two bearing shells or bearing shell halves are required for supporting the conventional cam carriers in the cylinder head of the internal combustion engine which results, on the one hand, in additional manufacturing and assembly costs and causes, on the other hand, increased friction losses in the event of an imperfect pairing of the two bearing shells or bearing shell halves.
To reduce the manufacturing complexity, the cam carrier may have a modular design and may be composed of the basic element and the at least one cam element. The basic element is essentially tubular and at least partially receives the basic camshaft. For this purpose, it encompasses the basic camshaft in the circumferential direction at least partially, preferably completely. The basic element preferably has an internal toothing which engages with an external toothing of the basic camshaft for holding the basic element on the basic camshaft in a rotationally fixed manner. The basic element can be completely drawn together with the internal toothing as a profile and can thus be easily and inexpensively produced. The at least one cam element is arranged on the basic element. The cam element is in particular formed as a cam disc. For example, the valve-actuating cam is embodied as a cam element. However, the cam element may alternatively also be another element, such as the shift gate, a spacer or a locking element. The cam element is preferably made of bearing steel, which is (fine-) punched and ground or reamed on its inner side. Identical or similar cam elements can be ground or reamed together, which allows for an efficient and economical production.
For example, DE 10 2009 022 657 A1 discloses a camshaft for an internal combustion engine. This camshaft consists of a basic shaft having at least one external toothing, and at least one cam carrier which is axially displaceably mounted on the basic shaft and has at least one inner toothing co-operating with the external toothing. The external toothing or the internal toothing is to be made of plastic, wherein the cam carrier made of plastic is molded around the cam elements. Therefore, a modular construction of the cam carrier and the cam elements is already described to some extent. However, the production cost is very high.